Fix ICE when applying test macro to crate root
This PR does a couple of things. First of all, I found [an ICE](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=a733a7f3d223e1a9712e44b571f3e5cf) that happens when applying `#![core::prelude::v1::test]` to the crate root. This is caused by the test macro not expanding to an item when `--test` isn't applied. For the crate root, that means it deletes the crate....
The fix now first does target checking, and only if the target is valid discards the item when `--test` isn't applied. The discarding is, I think, important for perf.
The problem with this PR is that it means that `#[test]` applied to structs previously would give no errors unless `--test` is applied! That sounds like a bug to me, but maybe we should crater run it just in case, since technically that's a breaking change. Errors in such items wouldn't be reported previously.
Also fixed a smol diagnostics bug with `#[bench]`'s error messages refering to `#[test]` accidentally.
r? noratrieb (since I already explained you a bunch, feel free to re-assign)
Fixes https://github.com/rust-lang/rust/issues/114920
Matthias Krüger
5019bdaefe